The main concerns other photographers have that causes them to disallow people "shooting over their shoulder" seem to be (these are REAL issues mentioned by working professionals on wedding photography forums):

Attending the wedding of a friend is a special and exciting event - and people often like to take snapshots to help them remember the day.

As long as it doesn't interfere with my job, I will often go a little out of my way so that other people's photos will turn out well. In the example at right, I was standing directly behind the group when I noticed they were about to take a picture. I moved to the side, took a photo of them, and then moved even further away so I wouldn't be in their picture.
 

With so many people at a wedding and so many pictures being taken throughout the day, it's a given that sometimes people will accidentally get in the way (even if they are trying to avoid the camera!). I work at minimizing these instances by waiting for people to walk past, or, letting them know I am about to take a picture. When accidents happen, I simply reshoot the image.
 

There are also times when people who are taking pictures will inadvertently get in my way - as the photos at right show. Several people had stepped into the aisle during the ceremony. As I mentioned above, I work to accommodate other photographers, and, since I had already gotten some close up images and nothing important was immediately scheduled, I was OK with them getting their photos in the aisle.

However, a surprise adjustment to the schedule happened: the kiss was early (middle of the ceremony instead of the very end). Not only that, but as the Bride and Groom were about to kiss, those in the aisle moved in even closer! By quickly moving myself across the back of the church I was still able to get a photo.

However, my job is to take the best possible pictures for the Bride and Groom, and there are very rare times when I will very politely ask a guest to move. For example, immediately after the surprise kiss, I slipped up the aisle and whispered to the guest, "I've been taking pictures from the back and it's a bit challenging to take the photos the Bride and Groom would like when the aisle is occupied, if it would be possible for your to take photos without being in the aisle I would be very grateful." I have found guests to be very polite, extremely accommodating to me, and easy to work with. Most guests don't even realize I am in the back taking pictures - and I have never, ever, had a guest purposefully get in the way of a picture.